Strategies to Induce Blood Vessel Ingrowth into Skin Grafts and Tissue-Engineered Substitutes

Skin is a multilayer organ consisting of several tissues and appendages residing in a complex niche. Adequate and physiologically regulated vascularization is an absolute requirement for skin homeostasis, regeneration, and wound healing. The lack of vascular networks and ischemia results in delayed wound closure. In addition, vascularization is critical for the prolonged function and survival of skin grafts and tissue-engineered skin substitutes. This study highlights the clinical challenges associated with the limited vascularization in the cutaneous wounds. Then, we highlight the novel approaches for the development of vascular networks in the skin autografts, allografts, and artificial substitutes. Also, the future directions to overcome the existing vascularization complications in skin grafting and synthetic skin substitutes are presented. Statement of Significance Delayed closure of large dermal wounds, such as burn injuries, results from the lack of vascular networks and ischemia. The amount of blood supply in the skin graft is the primary factor determining the quality of the transplanted grafts. The current skin grafts and their fabrication methods lack the appropriate features that contribute to the vascularization and integration of the wound bed and graft and adherence to the skin layers. Therefore, the new generation of skin grafts should consider advanced technologies to induce vascularization and overcome current challenges.

Evolution of ischemia and neovascularization in a murine model of full thickness human wound healing

Translation of wound healing research is limited by the lack of an appropriate animal model, due to the anatomic and wound healing differences in animals and humans. Here, we characterize healing of grafted, full-thickness human skin in an in vivo model of wound healing. Full-thickness human skin, obtained from reconstructive operations, was grafted onto the dorsal flank of NOD.Cg-KitW41J Tyr + Prkdcscid Il2rgtm1Wjl /ThomJ mice. The xenografts were harvested 1 to 12 weeks after grafting, and histologic analyses were completed for viability, neovascularization, and hypoxia. Visual inspection of the xenograft shows drying and sloughing of the epidermis starting at week four. By week 12, the xenograft appears healed but has lost 63.05 ± 0.24% of the initial graft size. There is histologic evidence of epidermolysis as early as 2 weeks, which progresses until week 4, when new epidermis appears from the wound edges. Epidermal regeneration is complete by week 12, although the epidermis appears hypertrophied. An initial increase of infiltrating immune mouse cells into the xenograft normalizes to baseline 6 months after grafting. Neovascularization, as evidenced by positive staining for the proteins human CD31 and alpha smooth muscle actin, is present as early as 2 weeks after grafting at the interface between the xenograft and the mouse tissue. CD31 and alpha smooth muscle actin staining increased throughout the xenograft over the 12 weeks, leading to greater viability of the tissue. Likewise, there is increased Hypoxia Inducible Factor 1-alpha expression at the interface of viable and nonviable tissue, which suggest a hypoxia-driven process causing early graft loss. These findings illustrate human skin wound healing in an ischemic environment, providing a timeline for use of full thickness human skin after grafting in a murine model to study mechanisms underlying human skin wound healing.

Adipose-derived stem cells improve full-thickness skin grafts in a rat model

To investigate the effects of heterologous adipose-derived stem cells (ADSCs) on autologous full-thickness skin grafts, we designed a first-intention healing model using Wistar rats. We harvested and sutured two full-thickness skin grafts in the dorsal recipient beds of 15 rats, randomized into three groups. In the treatment group, 1 × 10⁶ ADSCs resuspended in saline solution (200 μL) were administered subcutaneously to the skin graft. The control group received only saline solution subcutaneously, whereas the negative control group did not receive any treatment. Compressive dressings were maintained until postoperative day 5. The grafts were assessed by two observers, who checked for the presence of epidermolysis on day 14. Planimetry showed the relative areas of normal skin, redness, ulceration, and contraction. Graft samples were obtained on day 14 and stained with hematoxylin and eosin and Masson's trichrome. Epidermal analysis evaluated thickening, keratosis, acanthosis, hydropic degeneration, and inflammatory infiltrate. Dermal evaluation investigated the absence of hair follicles, granulation tissue formation, presence of inflammatory infiltrate, and collagen deposition. Immunohistochemistry was performed for dermal anti-VEGF and epidermal anti-Ki-67 staining. The ADSC group presented better macroscopic aspects, lower incidence of epidermolysis, and less loss of hair follicles. In addition, the ADSC group presented the lowest frequency of histopathological changes in the dermis and epidermis, as well as the largest subcutaneous and granulation tissue VEGF averages and the weakest Ki-67 staining of the epidermal basal layer. Subcutaneous administration of ADSCs may improve the integration of skin grafts, reducing the deleterious effects of ischemia and reperfusion injury.

Effects of topical copper tripeptide complex on wound healing in an irradiated rat model

OBJECTIVES

To evaluate clinical and immunohistopathological effects of topical glycyl-histidyl-lysine-copper (GHK-Cu) on in vivo irradiated rat wounds.

DESIGN

Animal model.

SETTING

Academic institution.

SUBJECTS AND METHODS

After dorsal irradiation and a 28-day recovery period, 2 × 8 cm cranially based dorsal flaps were created in Sprague-Dawley rats. Twice daily GHK-Cu gel (test) or aquaphilic ointment (control) was applied for 10 days. Animals were euthanized, digital images of flaps were taken, and harvested tissues were immunohistochemically stained for a vascular endothelium marker, caveolin-1, and vascular endothelial growth factor (VEGF). Digital image analysis was used for outcome measures. Unpaired t-tests were used for statistical analyses; significance of P < .01 accounted for multiple comparisons.

RESULTS

By digital analysis of clinical images, 13 test and 10 control animals showed mean ischemic areas of 5.0 cm(2) (SD = 0.9) for tests and 3.8 cm(2) (SD = 1.1; P = .011) for controls. Whole slide digitized images allowed quantification of caveolin-1-stained blood vessels and VEGF expression in fibroblasts at the interface of healing flaps. Caveolin-1 analyses showed a mean of 209.0 vessels (SD = 111.1) and a mean vessel luminal area of 525.7 um(2) (SD = 191.0) in tests and 207.4 vessels (SD = 109.4; P = .973) and 422.8 um(2) (SD = 109.7; P = .118) in controls. VEGF quantified as the percentage of pixels exceeding a colorimetric threshold, with higher fractions of positive pixels indicating more intense staining, showed a mean intensity score of 0.34 (SD = 0.19) in tests and 0.54 (SD = 0.41; P = .169) in controls.

CONCLUSIONS

Irradiated dorsal rat flaps treated with topical GHK-Cu gel demonstrated no difference in flap ischemia, blood vessel number or area, or VEGF expression compared to controls.